The conversion of guaiacol to catechol in high temperature water by catalysis of mineral hydrochloric acid is examined. The effects of pH and H-2 pressure are measured in the conversion of guaiacol. Hydrogen enhances the reaction dramatically. Moreover, low pH and high hydrogen pressure favor the reaction. The highest conversion of guaiacol and best yield of catechol of 99% and 89%, respectively, are achieved with 1 MPa hydrogen at pH = 1.8 for 3 h at 280 degrees C. Based on the experimental results and kinetics, possible reaction mechanisms are proposed. Besides ionic mechanism and water catalysis, hydrogen polarization also occurred without metal catalyst. (C) 2012 Elsevier B.V. All rights reserved.